This invention relates to structures that have quantum coherence, such as Josephson junctions, and more particularly to their application in superconducting quantum computing.
The quantum computer is rapidly evolving from a wholly theoretical idea to a physical device that will have a profound impact on the computing of tomorrow. A quantum computer differs principally from a conventional, semiconductor chip-based computer, in that the basic element of storage is a xe2x80x9cquantum bitxe2x80x9d, or xe2x80x9cqubitxe2x80x9d. A qubit is a creature of the quantum world: it can exist in a superposition of two states and can thereby hold more information than the binary bit that underpins conventional computing. One of the principal challenges in quantum computing is to establish an array of controllable qubits, so that large scale computing operations can be carried out. Although a number of different types of qubits have been created, it is believed that the practical realization of a large scale quantum computer is most likely to be achieved by harnessing the properties of superconducting junctions. It is in the superconducting regime that many materials display their underlying quantum behavior macroscopically, thereby offering the chance for manipulation of quantum states in a measurable way.
In 1962, Josephson proposed that non-dissipating current would flow from one superconductor to another through a thin insulating layer, see B. D. Josephson, Phys. Lett., 1:251, (1962). Since then, the so-called Josephson effect has been verified experimentally and has spawned a number of important applications of superconducting materials. In particular, it has been found that the thin insulating layer originally used by Josephson is an example of a general class of barriers known as weak links. These weak links are interruptions of the translational symmetry of the bulk superconducting material on the same scale as the coherence length of the material. Examples of weak links include the following: grain boundaries, insulating gaps, tunneling junctions, constrictions, and any locations where the amplitude of the order parameter of the superconductor is diminished. The Josephson effect has been generalized to all weak links in a superconductor. Therefore any small interruption of a superconducting material, or an interface of two different superconductors, behaves as a Josephson junction. Avoiding the formation of weak links where Josephson junctions are not intended can make the fabrication of devices based on superconducting components difficult.
Nevertheless, the Josephson junction has found practical application in a device known as a superconducting quantum interference device (SQUID). The current and voltage of a superconducting loop with two small insulating gaps behaves in a previously unexpected way that depends on the magnetic flux enclosed in the loop. SQUIDs are useful for sensitive measurement and in the creation of magnetic fields. For example, see chapter 1 of A. Barone and G. Paternò, Physics and Applications of the Josephson Effect, John Wiley and Sons, New York, (1982), which is incorporated herein by reference.
Two types of superconductors are regularly used nowadays: conventional superconductors and unconventional superconductors. The most important phenomenological difference between the unconventional superconductors and conventional superconductors is in the orbital symmetry of the superconducting order parameter. In the unconventional superconductors, the pair potential changes sign depending on the direction of motion in momentum space. This has now been experimentally confirmed; see e.g., C. C. Tsuei and J. R. Kirtley, Rev. Mod. Phys., 72, 969, (2000). The effects of this pairing were understood long before this experimental confirmation.
For example, it was discovered that in unconventional superconducting materials such as YBa2Cu3Ox (xe2x80x9cYBCOxe2x80x9d) that have orthorhombic crystal structures, there existed a significant subdominant order parameter mode that is spherical in momentum space (referred to as an s-wave); see K. A. Kouznetsov et al., Phys. Rev. Lett., 79, 3050, (1997).
The coherence length of an unconventional superconductor is not isotropic. In an orthorhombic superconductor, the coherence length in the c-axis direction is much less than in the a and b directions. Correspondingly, the critical current is much smaller in the c-axis direction. Furthermore, the coherence length in all directions of an unconventional superconductor is small enough for a weak link to form easily at any junction. Given that the Josephson effect is present in all weak links, the short coherence length poses a difficulty for forming devices that utilize unconventional superconducting materials.
Hence, superconducting single electron transistors (SET""s) have generally been made from conventional superconductors. Efforts to make them from unconventional superconducting materials have not been particularly successful. See, e.g., S. E. Kubatkin et al., JETP Lett., 63, 126-132, (1996) and A. Tzalenchuk, poster presentation at SQUID 2001, both of which are incorporated herein by reference. The oscillations of a SET made from an unconventional superconducting material would have only a single charge periodicity, not both a single and a double charge period. Both effects are useful in superconducting quantum computing, where a mechanism for controllable switching of supercurrent is important and where the supercurrent charge carriers are Cooper pairs. Thus there is a need for a controllable supercurrent switch that is based on an unconventional superconductor.
Other types of junctions are known, but suffer from deficiencies that prevent their use in superconducting quantum computing. For example, Racah et al., Physica C, 263, 218-224, (1996), incorporated herein by reference, teach a junction comprising: YBCO, aluminum oxide and silver, i.e., an unconventional superconductor and a normal metal, separated by an insulator. However, no Josephson effect (in which Cooper pairs tunnel) was observed in Racah et al.""s structures; instead only quasi-particle tunneling was found. Furthermore such a junction has a dimension on the order of tens of microns.
Furthermore, junctions known in the art are far larger in area than the mesoscopic devices to which they need to be attached. This is a severe limitation, as size is often an enabling feature in quantum computers built from superconducting material. Certain components must be mesoscopic. Therefore, to implement quantum computing structures, Josephson junctions between conventional and unconventional superconductors are necessary, and no junction in the prior art suffices.
In accordance with the present invention, a Josephson junction is presented. In some embodiments, the junction includes an unconventional superconductor, an intermediate material, and a conventional superconducting material. In some embodiments, the resulting junction is in the c-axis direction of an orthorhombic unconventional superconductor. Alternatively, the junction may be oriented so that supercurrent flows in a direction in the a-b plane. The junction may be oriented so that either the c-axis direction or a direction in the a-b plane are aligned with the rest of the junction.
The present invention involves a Josephson junction comprising, in sequence: a first superconducting material layer; an intermediate layer having a first area of overlap with the first superconducting material layer; and a second superconducting material layer having a second area of overlap with the intermediate layer; wherein an area of intersection of the first area of overlap and the second area of overlap is less than or equal to about 0.1 xcexcm2, and wherein the first superconducting material layer is in contact with a substrate. In a preferred embodiment, the first superconducting material is a crystalline material having a crystal structure with an a-axis, a b-axis, and a c-axis; such that the c-axis is normal to the substrate.
A Josephson junction according to the present invention may also be a ramp-type junction wherein a first surface of the first superconducting material is parallel with the substrate; and a second surface of the first superconducting material makes an angle of inclination of between 0xc2x0 and 90xc2x0 with a plane that is normal to the first surface, wherein the angle of inclination is measured in a sense exterior to said first superconducting material and wherein the first surface and the second surface share an edge.
The present invention also involves a superconducting structure comprising, in sequence: a substrate; a first superconducting material layer comprised of unconventional superconducting material and includes a first region and a second region wherein the first region is disjoint from the second region; an intermediate material layer; and a second superconducting material layer that includes an island, wherein the island has a first area of overlap with the first region and a second area of overlap with the second region of the first superconducting material layer; and wherein an electrode is capacitively coupled to the island so that supercurrent can flow coherently between the first region and the second region of the first superconducting material layer.
Josephson junctions according to embodiments of the present invention may be used in superconducting low inductance qubits (SLIQs) and in permanent readout superconducting qubits (PRSQs). They can form the basis of quantum registers, and can allow for parity keys or other devices made from conventional superconducting material to be efficiently coupled to qubits made from unconventional superconducting material. Further, embodiments of the invention may be applicable to any electronic device that utilizes a superconducting junction wherein coherent transport between unconventional superconductors is useful.
In some embodiments, an unconventional superconductor having non-zero angular momentum pairing is placed with its c-axis perpendicular to the substrate. A conventional superconductor having a dominant mode that has zero angular momentum pairing is placed above the unconventional superconductor. Another material such as a normal metal or an insulator separates the conventional and unconventional superconductors to form a heterostructure (or heterojunction) to which electrodes may be attached.
Multiple methods of fabrication are described. Relative sizes of physical parameters needed for operation are introduced. Different materials as intermediate layers are described. The usage of various embodiments of heterojunctions in quantum computing structures is outlined.